1. Field of the Invention
The present invention relates to resistor substrates for use in variable resistors and the like, and more particularly, relates to a resistor substrate and a manufacturing method thereof, the resistor substrate having a resistor layer embedded therein so that a surface thereof is exposed.
2. Description of the Related Art
In a related resistor substrate, a resistor layer having a predetermined pattern is provided on a surface of a molded base, and electrode layers are provided under two end portions of this resistor layer. In the case of a variable resistor, terminals are connected with the respective electrode layers, a sliding contact is provided which slides on a surface of the resistor layer located between the electrode layers, and the change in voltage can be detected between each terminal and the sliding contact in accordance with a sliding position thereof.
The related resistor substrate as described above is generally manufactured by the following steps. In a first step, a resistor paste is formed by mixing carbon black, pulverized carbon fiber, and a thermosetting binder resin such as a polyimide resin with a solvent, and this resistor paste is screen-printed on a transfer sheet formed of brass.
In a second step, the resistor paste is dried, for example, at 200° C. for approximately 30 minutes, thereby removing the solvent. In addition, on the resistor paste thus dried, the steps of screen printing and drying described above are repeatedly performed, thereby obtaining a resistor layer composed of at least two films each formed of the dried resistor paste.
In a third step, an electrode paste formed of a thermosetting binder resin and powdered silver dispersed in a solvent is screen-printed on the resistor layer and is then dried, for example, at 200° C. for 30 minutes, followed by further drying at 260° C. for approximately 30 minutes, so that the solvent is removed.
In addition, in the third step, heating is performed, for example, at 380° C. for approximately 70 minutes so that the binder resin used for the resistor paste and the binder resin used for the electrode paste are simultaneously heat-cured, thereby forming the resistor layer in which the carbon is dispersed in the resin and the electrode layers in which the powdered silver is dispersed in the resin.
In a fourth step, the transfer sheet provided with the resistor layer and the electrode layers formed in that order is placed in a mold, and after an epoxy resin or the like is injected into a cavity of the mold and is then cooled, the transfer sheet is removed.
Accordingly, at the surface of the substrate, the resistor layer is exposed, and the electrode layers are disposed under this resistor layer, so that the resistor layer and electrode layers are formed in the substrate.
However, in the resistor substrate manufactured by the related method described above, the powdered metal, that is, the powdered silver, dispersed in the electrode layers is likely to diffuse into the resistor layer, and the powdered silver is likely to ooze to the surface of the resistor layer.
The reason for this is that, in the related manufacturing method, since after the resistor layer and the electrode layers are patterned, the binder resins of both types of layers are simultaneously heat-cured by the heating described above, the powdered silver in the electrode layers is liable to ooze into the resistor layer when the resins are softened during the step in which the binder resins are cured. In particular, since the same binder resin has been used for the resistor layer and the electrode layers, the resin in both types of layers are simultaneously softened in the heating step, and at this stage, the powdered metal (powdered silver) in the electrode layers is likely to ooze into the resistor layer.
In addition, when the powdered silver used as the powdered metal oozes to the surface of the resistor layer, the silver may be corroded (sulfurated or the like), or migration thereof may occur to cause short-circuiting between adjacent patterns.
In addition, in a variable resistor, the change in resistance is detected when a sliding contact slides on a surface of a resistor surface located between electrode layers; however, while sliding, the sliding contact may slide on a region (multilayer pattern) in which the resistor layer is formed on the electrode layer in some cases. When the powdered silver oozes into the resistor layer in this region as described above, since the hardness of silver is lower than that of carbon particles (particularly, carbon fiber), the strength of the resistor layer in that region is decreased, and as a result, the resistor layer is liable to be worn by the sliding movement of the sliding contact.
In order to suppress the exposure of the powdered silver at the surface of the resistor layer, as described in the second step, the pattern printing of the resistor paste and the drying thereof are repeated at least twice in the past so as to increase the thickness of the resistor layer. However, in this case, the number of the pattern printing of the resistor paste is increased, and the productivity is decreased, resulting in high manufacturing cost. In addition, even when the thickness of the resistor layer is increased, it has been difficult to substantially prevent the powdered metal (powdered silver) from oozing into the resistor layer during heat-curing.